If the water drains out for any reason, it will cause a fire in the fuel rods, as the zirconium metal jacket on the outside of the fuel rods could very well catch fire within hours or days after being exposed to air. See this, this, this and this. (Even a large solar flare could knock out the water-circulation systems for the pools.)

The pools are also filling up fast, according to the Nuclear Regulatory Commission:

The reactor operators have squeezed spent fuel more tightly into the pools, raising the heat load and, according to some analyses, raising the risk of fire if the pools were ever drained.

Indeed, the fuel pools and rods at Fukushima appear to have “boiled”, caught fire and/or exploded soon after the earthquake knocked out power systems. See this, this, this, this and this.

Robert Alvarez – a nuclear expert and a former special assistant to the United States Secretary of Energy – notes that there have also been many incidents within the U.S. involving fuel pools:

Even though they contain some of the largest concentrations of radioactivity on the planet, U.S. spent nuclear fuel pools are mostly contained in ordinary industrial structures designed to merely protect them against the elements. Some are made from materials commonly used to house big-box stores and car dealerships.

***

All spent fuel pools at nuclear power plants do not have steel-lined, concrete barriers that cover reactor vessels to prevent the escape of radioactivity. They are not required to have back-up generators to keep used fuel rods cool, if offsite power is lost.

***

For nearly 30 years, Nuclear Regulatory Commission (NRC) waste-storage requirements have remained contingent on the opening of a permanent waste repository that has yet to materialize. Now that the Obama administration has cancelled plans to build a permanent, deep disposal site at Yucca Mountain in Nevada, spent fuel at the nation’s 104 nuclear reactors will continue to accumulate and are likely remain onsite for decades to come.

According to Energy Department data:

The spent fuel stored at 28 reactor sites have between 200-450 million curies of long-lived radioactivity;

19 reactor sites have generated between 100-200 million curies in spent fuel; and,

24 reactor sites have generated about 10-100 million curies.

Over the past 30 years, there have been at least 66 incidents at U.S. reactors in which there was a significant loss of spent fuel water. Ten have occurred since the September 11 terrorist attacks, after which the government pledged that it would reinforce nuclear safety measures. Over several decades, significant corrosion has occurred of the barriers that prevent a nuclear chain reaction in a spent fuel pool — some to the point where they can no longer be credited with preventing a nuclear chain reaction. For example, in June 2010, the NRC fined Florida Power and Light $70,000 for failing to report that it had been exceeding its spent fuel pool criticality safety margin for five years at the Turkey Point reactor near Miami. Because of NRC’s dependency on the industry self-reporting problems, it failed to find out that there was extensive deterioration of neutron absorbers in the Turkey Point pools and lengthy delays in having them replaced.

There are other strains being placed on crowded spent fuel pools. Systems required to keep pools cool and clean are being overtaxed, as reactor operators generate hotter, more radioactive, and more reactive spent rods. Reactor operators have increased the level of uranium-235, a key fissionable material in nuclear fuel to allow for longer operating periods. This, in turn, can cause the cladding, the protective envelope around a spent fuel rod, to thin and become brittle. It also builds higher pressure from hydrogen and other radioactive gases within the cladding, all of which adds to the risk of failure. The cladding is less than one millimeter thick (thinner than a credit card) and is one of the most important barriers preventing the escape of radioactive materials.

***

I co-authored a report in 2003 that explained how a spent fuel pool fire in the United States could render an area uninhabitable that would be as much as 60 times larger than that created by the Chernobyl accident. If this were to happen at one of the Indian Point nuclear reactors located 25 miles from New York City, it could result in as many as 5,600 cancer deaths and $461 billion in damages.

The U.S. government should promptly take steps to reduce these risks by placing all spent nuclear fuel older than five years in dry, hardened storage casks — something Germany did 25 years ago. It would take about 10 years at a cost between $3.5 and $7 billion to accomplish. If the cost were transferred to energy consumers, the expenditure would result in a marginal increase of less than 0.4 cents per kilowatt hour for consumers of nuclear-generated electricity.

Another payment option is available for securing spent nuclear fuel. Money could be allocated from $18.1 billion in unexpended funds already collected from consumers of nuclear-generated electricity under the Nuclear Waste Policy Act to establish a disposal site for high-level radioactive wastes.

This situation cannot be blamed on the nuclear industry alone (which wouldn’t exist without government subsidization of the nuclear industry). The U.S. government promised to come up with a permanent storage solution more than a decade ago, but has failed to do so. As nuclear affairs chief Terry Pickens for Xcel Energy correctly says:

We were able to get it where we thought we could make it to 1998, and they are still not performing. And now we still want to refuel and operate our reactors, so we have to make more space in the pools.

Most of the plants now operating were designed to store fuel for only a few years, because engineers expected that it would either be recycled or buried. The Energy Department was supposed to begin accepting fuel for burial in 1998 but has not yet done so.

***

The Nuclear Regulatory Commission has repeatedly said that cask storage and pool storage are equally safe. On March 14, the commission’s chairman, Nils J. Diaz, told reporters that the pools ”are not easily breached structures.”

After an attack, they would be very easy to cool, he said. ”You get a couple of fire hoses, and spray them, and you have many, many hours,” he said, before there could be any radiological release, giving officials time to contain the problem.

That isn’t working out so well at Fukushima.

Single Pool Near Major American City Holds More Cesium than Fukushima, Chernobyl and all Nuclear Tests … Combined

airplane aluminum at 700 miles per hour on a width of 100 feet
vs.
RPG uranium depleted amunition at 1500 miles per hour on a width of 1 foot loaded with high velocity explosives
Dr. Benway, I hope you,re not a physics professor.

WASHINGTON—Structures that house reactor fuel at U.S. nuclear power plants would protect against a release of radiation even if struck by a large commercial jetliner, according to analyses conducted over the past several months by the Electric Power Research Institute (EPRI).

The independent analyses were conducted at the request of the Nuclear Energy Institute (NEI).

The computer analyses, which cost more than $1 million, are summarized in a report entitled, “Deterring Terrorism: Aircraft Crash Impact Analyses Demonstrate Nuclear Power Plant’s Structural Strength.” A summary of the study’s findings is accessible on NEI’s web site at http://www.nei.org.

Yes, it's called verification and validation, a formal process of verifying designs.

I think the Crystal River plant with its cracked containment building might not hold up as well, and I think it is extremely unlikely that these simulations have modeled the performance of cracked containment buildings. So, what the NEI have created is a mathematical lie, rather than the usual verbal lies. Fail.

That would be necessary to validate the model, yes. Lacking empirical verification, there is no proof, just a hypothetical mathematical model. (Oh, and it's secret, so we can't even check the basic assumptions)

Right. The nuclear trade lobby commissioned this from the electric power industry's pet research lab. I bet if they asked for a study of a nuclear plant surviving a magnitude 9 earthquake and tsunami it would turn out exactly the same.

LOL you conspiracy buffs are all the same!
You ask for proof. Then when provided with proof, you ask for proof for the proof. Repeat ad infinitum.
Its like when a five year old starts asking you "why?"

Oh Geoge W. dear! You ARE such a purveyor of pessimism porn, don't you think? We all know that a good dosing of radiation is GOOD for a body. I'm almost sad that I don't have to walk through the TSA scanners due to my celebrity status. I'm so disasppointed when I walk by the little people on the way to the private jet. So I'm hoping for a "little spill" out Boston way sometime. It will just make folks glow!

Iodine is a scam. It only protects you from one of the isotopes. What about the others? We are surrounded by snake oil salesman.

If you want to survive a nuclear disaster, you cannot be immediately downwind from it. If you want to survive a nuclear blast, you cannot be within the blast area or immediately downwind from it. Nuclear blasts are preferrable, since most of the material is "burned up" during the event. The fallout that remains quickly loses its lethal dose too. Nuclear disasters on the other hand spew lethal dose radioactive particles for extended periods.

The bottom line is this: Save your money for a quality mask, filters, and other protective gear. Build a house with a "cold storage" room. Make sure you can survive within it for at least a month. Do not live immediately downwind from a nuclear facility, do not share a water source with one, and be 30 miles from any high value target. Make sure you have a geiger counter that is running 24/7 that will also give audible alarms when there's a spike in alpha, beta, or gamma particles.

I think the money spend chasing down a few Afghan guerrillas could be better spent protecting the American Public from a threat that is more real and closer to home. Of course the lobby that represents the nuclear power industry would not support such a move, even though the American Pubic wants it.

What is wrong with this picture? What is wrong with our congressmen. It will take a disaster to get their attention. I expect I will live to see an American disaster from nuclear fallout, either from our own or the effects from Japan.